WATER police who rushed to investigate reports of an overturned boat with people in the water off a popular Sydney beach instead discovered two immense great white sharks feeding on the remains of a whale.

The largest of the sharks was estimated to be six metres long, and the smaller shark "just'' 4.5 metres long.

The sharks were feeding on the floating blubber only a few hundred metres off Whale Beach on Sydney's northern beaches on Saturday afternoon.

Footage has now emerged of the sharks feeding on the head of the whale further out to see.

"The bigger of the two sharks was the biggest thing apart from a full grown humpback I've ever seen in the water,'' one of the Broken Bay water police officers said.

"It was about six metres long, it came up beside our boat many times and that's a reliable estimate of its size, it was a very very big shark.

"They were really getting stuck into the whale blubber, it was about three square metres at the start but a lot smaller by the time we left them.''

The sharks were last seen just a few hundred metres off the southern end of Whale Beach on dusk.